Mutual information between a modulation-constrained input and its output through Gaussian channels is referred to as the mutual information per modulation symbol or the modulation-constrained capacity. Mutual information has been studied as a performance metric for Gaussian channels. However, it is difficult to compute the mutual information for multiple-input multiple-output (MIMO) Gaussian channels. In addition, a single-layer physical layer abstraction method may not capture the true gain of a nonlinear receiver.